This invention relates to methods of detecting leaks in a fluid flow path, particularly when said fluid comprises a mixture of a condensable gas and an incondensable gas, and more particularly still, where said fluid flow path is at the outlet to a steam turbine.
Large condensing steam turbines used to produce power are responsible for producing a large proportion of the carbon dioxide emitted by the UK. As such, maximising efficiency of these turbines is very important both in terms of fuel cost to their operators and in relation to their environmental impact.
Turbines work by extracting energy, in the form of heat, from a working fluid and converting that heat energy into useful mechanical energy, for example, providing rotary power to an electrical generator. The more heat energy that is extracted from the fluid by the turbine, the more rotational energy it produces. As such, increasing the pressure difference across the turbine increases the energy extracted by the turbine, which results from a greater utilisation of the working fluid heat by the turbine. A major constraint on the efficiency of this type of system is the pressure of the steam turbine exhaust. Minimising this pressure results in the extraction of a greater amount of energy from the steam by increasing the pressure difference across the turbine. There is typically a water-cooled condenser situated at the turbine exhaust, which both condenses the steam and lowers the exhaust pressure. The condenser typically comprises a plurality of coolant tubes through which the cooling water flows, the heat from the steam being transferred to the tubes. The pressure of the exhaust within the condenser is typically known as the back-pressure.
The back-pressure may be elevated by a number of factors which include: high cooling water temperature within the condenser; low cooling water flow; low heat transfer rate between the turbine exhaust steam and the cooling water; and the presence of air in the steam being condensed.
The temperature of the steam in a given design of condenser is fixed by various parameters including cooling water temperature, cooling water flow and the heat transfer rate of the cooling water tubes, in combination with the mass flow rate of steam. The back-pressure in the absence of air is that corresponding to the saturation vapour pressure of the steam at that particular fixed temperature, and as such, any air that is present will create a partial pressure which must to be added to the saturation vapour pressure resulting in a greater back-pressure.
In addition, as the steam condenses, any incondensable air may surround the cooling tubes causing an effect known as air blanketing. This effect reduces the heat transfer rate of the tubes, hence the steam temperature is greater and, as such, the back-pressure is further increased.
Some air leakage into these complex systems is inevitable and so air is continuously removed from the condenser by vacuum pumps. If these fail or are overwhelmed by air due to excessive leakage into the condenser, the partial pressure of air steadily rises and may eventually result in damage to the turbine or a costly shutdown of the system.
When excess air is present in a condenser, the vacuum pumps reach equilibrium where the suction pressure corresponds to the mass flow rate of the air passing through the pump.
Although hereinbefore the condenser has been referred to, the portion of the system at vacuum comprises the condenser, turbine exhaust ducts, numerous ancillary vessels, vacuum pumps, connective pipework and instrumentation.
Typically, the vacuum pumps on a 500 MW turbo-generator unit can handle about 160 kg/hr of air before the back-pressure starts to rise. This equates to a single hole of approximately 10 mm2 in a system that might comprise 1000 m of piping and 2400 m2 of vessel surface area. It is therefore unsurprising that a major cause of high back-pressure is often excess air leakage.
To reduce the high back-pressure it is necessary to determine if the cause is the presence of excess air in the system as opposed to the other factors mentioned above and if so to identify whether it is due to poor vacuum pump performance or high air ingress. If high air ingress is the cause, it is necessary to find the source(s) of the leaks and quantify them, so that they may be repaired, or sealed, as needed.
Various systems are available to monitor the flow of air to the vacuum pumps. Typically, the devices fitted to UK power stations are unreliable, require frequent re-calibration and have limited operating range. They can only be used in pre-installed locations and can only handle a limited range of air/steam ratios.
Numerous techniques exist to find air leaks into a vacuum system, including smoke generators, ultrasonic noise detectors and tracer gas systems. All are capable of finding leaks and giving the order of magnitude but none can accurately measure the leakage rate.
It is an object of the present invention to ameliorate at least some of the hereinbefore-described disadvantages.